An investigation is proposed to determine the effects of light and bleached pigment levels on the adaptive responses of vertebrate photoreceptors. It is well know that there exists a relationship between rhodopsin concentration and sensitivity in rod dark adaptation, and to each level of bleached pigment there corresponds a light background which are equivalent with respect to threshold. While the major part of adaptation occurs in photoreceptors, the effects of light and bleaching on photoreceptor membrane potential and conductance over the adaptive range of the cell are unknown. An investigation to determine these effects is proposed here. Membrane potential and conductance will be measured intracellularly and quantified in relation to light backgrounds and bleaching. This will be done in particular for the socalled slow, "photochemical" phase of dark adaptation. A systematic comparison will be made of eyecup, isolated retina (particulary in relation to bleaching) and single photoreceptors (especially as regards membrane conductance). These studies will be supplemented by extracellular recording of membrane current. Thus, we shall characteriz neural and photochemical adaptation in terms of the electrophysiological parameters and the response kinetics of photoreceptors